Fabrication of electronic circuit elements using liquid deposition techniques is of profound interest as such techniques provide potentially low-cost alternatives to conventional mainstream amorphous silicon technologies for electronic applications such as thin film transistors (TFTs), light-emitting diodes (LEDs), RFID tags, photovoltaics, etc. However the deposition and/or patterning of functional electrodes, pixel pads, and conductive traces, lines and tracks which meet the conductivity, processing, and cost requirements for practical applications have been a great challenge. Silver is of particular interest as a conductive element for electronic devices because silver is much lower in cost than gold and silver possesses much better environmental stability than copper.
U.S. Pat. No. 7,270,694, which is hereby incorporated by reference herein in its entirety, discloses a process comprising reacting a silver compound with a reducing agent comprising a hydrazine compound in the presence of a thermally removable stabilizer in a reaction mixture comprising the silver compound, the reducing agent, the stabilizer, and an optional solvent, to form a plurality of silver-containing nanoparticles with molecules of the stabilizer on the surface of the silver-containing nanoparticles.
U.S. Pat. No. 7,494,608, which is hereby incorporated by reference herein in its entirety, discloses a composition comprising a liquid and a plurality of silver-containing nanoparticles with a stabilizer, wherein the silver-containing nanoparticles are a product of a reaction of a silver compound with a reducing agent comprising a hydrazine compound in the presence of a thermally removable stabilizer in a reaction mixture comprising the silver compound, the reducing agent, the stabilizer, and an organic solvent wherein the hydrazine compound is a hydrocarbyl hydrazine, a hydrocarbyl hydrazine salt, a hydrazide, a carbazate, a sulfonohydrazide, or a mixture thereof, and wherein the stabilizer includes an organoamine.
Silver nanoparticles have also been prepared, for example as described in U.S. Pub. No. 2007/0099357 A1, incorporated by reference herein in its entirety, using 1) amine-stabilized silver nanoparticles and 2) exchanging the amine stabilizer with a carboxylic acid stabilizer. However, this method typically requires a carboxylic acid with a carbon chain length greater than 12 carbon atoms to afford sufficient solubility for solution-processing. Due to the high boiling point of such long-chain carboxylic acids and the strong bond between the carboxylic acid and silver nanoparticles, the annealing temperature required for obtaining conductive silver films is typically greater than 200° C.
While currently available methods for preparing conductive elements for electronic devices are suitable for their intended purposes, there remains a need for an method suitable for preparing conductive structures with a thickness of several micrometers and a low annealing temperature, wherein the metal nanoparticles used to prepared the conductive structure possess an increased shelf life. Although some specialty plastic substrates can withstand annealing temperatures of 250° C., most plastic substrates cannot and thus, dimensional stability is still an issue. Moreover, low cost plastic substrates favor an annealing temperature below 150° C. Further, a need remains for lower cost, environmentally safe methods for preparing liquid processable, stable metal nanoparticle compositions that are suitable for fabricating electrically conductive elements of electronic devices and have an increased shelf life.